Bigbwii - sorry for the delay in replying.
A small stomach is obtained through a combination of diet, exercise and abdominal exercise.
Obviously a sensible diet is necessary but myself and the other appointed ab gods still eat lot's - in the case of Topher and Jonathan I don't think they ever stop eating. I'm sure Alex eat's a hell of a lot too to maintain all that muscle mass.
Listen to your body - eat when hungry. Some days I really pig out. I haven't really ate too much the last couple of days at work - too many distractions made me forget about eating.
Eat more smaller meals rather than fewer larger meals. This keeps your metabolism ticking over and you don't use up most of your energy digesting your food. I am strong believer in grazing, in 'eat light, eat often' and all that.
You've got to get your bodyfat down to get a small stomach, especially if you want the abs to show. Obviously low bodyfat is achieved through diet and also as a result of cardio burning up fat.
Obviously you don't want to do too much cardio as you have stated you want to maintain your mass. However it is still possible to do some cardio and maintain most of your mass - obviously you would still be lifting nearly as often as you usually do, so you won't lose too much mass. You will lose a lil ( as Nat would say ) mass, but will gain all round defination so would look better all round, not just on your stomach.
By cardio I mean proper cardio. Too many people do it tokenly and kid themselves they have had a cardio workout. You want to be sweating, breathing hard, your heartbeat to be fast and hurtin basically. Obviously this is not wise if you are not used to cardio - not sure if you do any cardio at present? - so start off easy and get your cardio fitness levels up. Ultimately tho, your intention should be to do hard cardio.
If you want the sixpack, obviously you have to train the ab's as you would any other muscle group, and specifically concentrate on them. Don't do zillions of reps as that is a complete waste of time - you don't for your other muscles, so same principle for your abs. It is very important to vary the stomach exercises you do to target the overall abs, upper abs, lower abs, obliques etc etc. There are numerous ab exercsises you can get from magazies, books and the web. Do train your abs slowly and concentrate on contracting and squeezing the abs which helps to give a better 'burn'.
That said, I believe Jonathan has achieved his stomach through very little cardio or specific ab training.
Of course genetics plays a part, with some of us more lucky than others. What it comes down to is dedication, especially to be able to constantly maintain a flat stomach.
Hope that helped.